This invention relates generally to partial reflectors and, more particularly, to partial reflectors that include multi-layer, thin-film coatings deposited on substrates.
Partial reflectors, which sometimes are referred to as half mirrors or beamsplitters, are used in numerous applications. Such reflectors function both to reflect and to transmit light, in prescribed relative proportions. It usually is desirable to control not only the relative proportions of reflected and transmitted light, but also the reflector's efficiency, which represents the sum of its reflectance and transmittance. Whatever light is not reflected or transmitted is absorbed, which generally will heat the reflector.
In many optical applications, e.g., laser machining, excessive heating of the partial reflector can be particularly disadvantageous. Consequently, efficiencies near 100% are desired for such applications. In other optical applications, however, e.g., imaging and information display applications, achieving a high efficiency generally is considered to be less important than is achieving a desired ratio of reflectance and transmittance.
Reflectors and partial reflectors used in the past frequently have incorporated simple thin-film metallic coatings of silver or aluminum deposited on transparent glass or plastic substrates. Common household mirrors, for example, incorporate such coatings on the rear surfaces of such substrates, to protect the coatings from undesired environmental factors such as mechanical abrasion, oxidation and corrosion. In some applications, however, the metallic coating must be deposited on an exposed, front surface of the substrate. In those applications, rhodium, chromium and nickel-chromium alloys can be substituted for silver or aluminum, to provide better resistance to such environmental factors. Such thin metallic films sometimes can be protected by thin-film overcoats of silicon oxide or silicon dioxide, but these overcoats can affect the reflector's reflectance.
Partial reflectors based on simple thin-film metallic coatings cannot be readily configured to allow for independent selection of the reflectance, transmittance and efficiency. Basically, insufficient degrees of freedom are available to optical designers of such partial reflectors.
As an example, FIG. 1 shows the relationship between reflectance and transmittance for a number of partial reflectors having the form of thin-film coatings of aluminum deposited on transparent glass substrates, wherein the coatings for the reflectors range in thickness from 1.0 nanometer to 20 nanometers. It will be noted that increases the coating's thickness increases the partial reflector's reflectance and decreases its transmittance. It also will be noted that the partial reflectors generally exhibit higher reflectance in red wavelengths than in blue wavelengths and that the reflectors generally have efficiencies in the range of 80 to 90%.
Further, FIG. 2 shows this same fixed relationship between reflectance and transmittance for partial reflectors in the form of thin-film coatings of chromium deposited on transparent glass substrates. It will be noted that the spectral responses of the chromium reflectors are substantially more uniform, but that they have efficiencies of only about 60% in the middle of the depicted range. Changing the substrates from glass to polymethyl methacrylate has little effect on the depicted relationships.
Partial reflectors also can be formed by depositing thin-film dielectric coatings onto transparent substrates. For example, a partial reflector incorporating a single film of dielectric material (e.g., titanium dioxide, with a refractive index at 550 nanometers of about 2.34, deposited on a glass substrate) can exhibit about 33% reflectance and 65% transmittance, thus providing an efficiency of about 98%. Partial reflectors incorporating multiple layers of alternating high-index and low-index dielectric materials can exhibit higher reflectance, with greater spectral uniformity. Again, efficiencies approaching 100% can be achieved.
FIG. 3 depicts the reflectance and transmittance levels of a partial reflector incorporating a nine-layer dielectric coating. The coating incorporates an alternating stack of titanium dioxide and silicon dioxide deposited on a glass substrate. It will be noted that the depicted reflectance and transmittance levels exhibit pronounced maxima and minima as a function of wavelength, with an overall efficiency of about 73%. Multi-layer dielectric coatings of yet greater complexity are considered necessary to produce more uniform reflectance and transmittance spectra. This, of course, can lead to higher costs and manufacturing difficulties.
Other partial reflectors used in the past have included a three-layer stack deposited on a transparent substrate, in which the stack incorporates an intermediate metal layer (e.g., titanium) sandwiched between an inner layer formed of a material having a relatively high refractive index (e.g., titanium dioxide) and an outer layer formed of a material having a relatively low refractive index (e.g., silicon dioxide). Although the configuration of this partial reflector generally provides for greater independent selectability of reflectance, transmittance and efficiency than do the configurations of the partial reflectors described briefly above, it is considered unduly difficult and costly to manufacture.
It should, therefore, be appreciated that there is a need for an improved partial reflector configured to provide selected, spectrally uniform levels of reflectance, transmittance and efficiency, without being unduly difficult and costly to manufacture. The present invention fulfills this need and provides further related advantages.